1. Field of the Invention
The present invention relates to a control apparatus for an engine, including a supercharger to increase the pressure of intake air of the engine, an intake-air amount regulating valve to regulate an intake air amount in the engine, and an exhaust gas recirculation apparatus to circulate part of exhaust gas of the engine back to the engine, the control apparatus being arranged to control them according to an operating condition of the engine.
2. Related Art
Conventionally, a technique of the above type is employed in a vehicle engine, for example. An exhaust gas recirculation (EGR) apparatus is arranged to introduce part of exhaust gas after combustion, which is discharged from a combustion chamber of an engine to an exhaust passage, into an intake passage through an EGR passage so that the exhaust gas is mixed with intake air flowing in the intake passage and flows back to the combustion chamber. An amount of EGR gas flowing in the EGR passage is regulated by an EGR valve provided in the EGR passage. This EGR can reduce mainly nitrogen oxide (NOx) in the exhaust gas and improve fuel consumption during a partial load operation of the engine.
Exhaust gas from the engine contains no oxygen or is in an oxygen lean state. Thus, when part of the exhaust gas is mixed with the intake air by EGR, the oxygen concentration of the intake air decreases. In a combustion chamber, therefore, fuel burns in a low oxygen concentration. Thus, a peak temperature during combustion decreases, thereby restraining the occurrence of NOx. In a gasoline engine, even when the content of oxygen in intake air is not increased by EGR and a throttle valve is closed to some degree, it is possible to reduce pumping loss of the engine.
Herein, recently, it is conceivable to perform EGR in the entire operating region of the engine in order to further improve fuel consumption. Realization of high EGR is thus demanded. To realize the high EGR, it is necessary for conventional arts to increase the internal diameter of an EGR passage or increase the opening area of a flow passage provided by a valve element and a valve seat of an EGR valve. That is, an EGR valve has to be increased in size.
Meanwhile, it is well known that an engine including a supercharger is also provided with an EGR apparatus. JP-A-2012-7547 discloses this type of an engine. This engine includes a supercharger consisting of a turbine provided in an exhaust passage and a compressor provided in an intake passage and driven by the turbine. Further, an EGR passage is placed between a downstream side of the turbine in the exhaust passage and an upstream side of the compressor in the intake passage and an EGR valve is provided in the EGR passage (a low-pressure loop type EGR apparatus).
The above type of the engine has a relatively long path of the intake passage from an outlet of the EGR passage to a throttle valve. During deceleration of the engine, therefore, as shown in FIG. 25A, even a throttle opening degree (position) is closed to a fully closed position (time t1) and the EGR valve is immediately closed in sync with rapid reduction of a request EGR rate, a large amount of EGR gas is apt to stay in the intake passage between the outlet of the EGR passage and the throttle valve. Thus, such residual EGR gas is mixed in intake air, causing a delay in decreasing an EGR rate from the start of engine deceleration (EGR attenuation delay) as shown in FIG. 25B. As a result, the EGR rate in the intake air taken in the combustion chamber becomes excessive and misfire of the engine during deceleration may be caused. This shows a tendency that as the engine is decelerated from a higher supercharged pressure or as an engine rotation speed becomes lower, the time of EGR attenuation delay is longer. FIGS. 25A and 25B are time charts respectively showing behaviors of a throttle opening degree and EGR rate before and after engine deceleration.
Therefore, the engine disclosed in JP-A-2012-7547 is provided with a fresh-air bypass passage for introducing fresh air into an intake passage downstream of the throttle valve and a bypass valve placed in the fresh-air bypass passage. This engine is configured to control the bypass valve to an opening side and the throttle valve to a closing side when the request EGR rate of the engine rapidly decreases. Accordingly, in the case where the request EGR rate rapidly decreases at the time of engine deceleration, fresh air is introduced from the fresh-air bypass passage to the intake passage, thereby scavenging EGR gas remaining in the intake passage and mixing the EGR gas flowing in the intake passage downstream of the throttle valve with fresh air to early attenuate the EGR rate.